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How to Install Glass Block Windows in Your Basement

Tom Lohr is an avid home improvement enthusiast. He prefers to spend the money he saves on new tools and gardening supplies.

Yeah, my windows were that bad.

Yeah, my windows were that bad.

Time for an Upgrade

If you live in an older house east of the Mississippi, you probably have a basement. Basements are great for lots of domestic-related activities. They are usually the laundry room or workshop and crafts center. It's a great place to store your lawnmower for the winter and seasonal clothing. Unfortunately, basements are usually dark, dank, and a source of cold air leaks during the winter.

While keeping your basement dry can involve a considerable amount of time, effort, and expense, there is one upgrade you can do yourself that will help insulate your basement and allow for some ventilation to help with the dankness.

Many basements have glass block windows with smaller casement windows embedded. They provide good window insulation when it is cold and some airflow when it is wet or hot out. But not all basements enjoy the benefits of glass block windows.

Just as many have pane windows, most of those are single pane, and many of those are in rough shape. Not only do they let in cold air and offer no ventilation, but they are easily broken and allow an easy access point into your home for burglars.

If you have paned windows in your basement, you will benefit from replacing them with glass block units. It's not as hard as you might imagine and can easily be accomplished in an afternoon. If your basement is ready for a window upgrade, it's a perfect DIY project.

Let's Get to It

You can build your own glass block window, block by block, purchasing the blocks separately, but that requires advanced skills and is not an afternoon DIY project. These instructions are for installing the pre-made glass block windows that you can get at the big box home improvement store for just over $100 each.

Also note that while you can absolutely accomplish this project by yourself, it is much easier and efficient if you have someone to help you. Your helper doesn't need to be Bob Villa, just someone to hold the window when needed and help you place it. The window units are not super heavy, but they're not light either; a person of average size and strength can handle one.

Materials Needed

  • Pre-assembled glass block window
  • Structural repair mortar
  • Mixing bucket or tray
  • Mixing tool (a trowel is fine)
  • Putty knife
  • Hammer
  • Pry bars
  • Utility knife
  • Nitrile gloves
  • Work gloves
  • Small level
  • Wood shims
  • Can of expanding insulation foam
  • All-purpose sealant

Step 1: Measure Your Window Opening

For this measurement, you want to measure the absolute opening. Forget adding in frames or other things in your window; they will all be removed. That means you are likely measuring from block to block or brick to brick in each opening.

Once you have your numbers, check what they have at your local hardware store. There is a lot of wiggle room for glass block windows, so it doesn't have to be a perfect fit, but as close as possible. Most hardware stores carry two or three of the most popular sizes.

Most also come with the same type of screened casement window embedded, but you can get a glass block window without any openings if desired. Just remember you won't get the benefit of natural ventilation if you choose one.

Step 2: Select Your Window

With measurements in hand, head to the hardware store and see what they have or check their website. There are only a few standard sizes, but there are several styles of obscured glass blocks used. Pick one that fits your needs and taste and lug it home. If semi-heavy objects are an issue for you, bring a helper.

Wooden frame that has to be removed.

Wooden frame that has to be removed.

Step 3: Prepare the Opening

Pick a day when no rain is predicted. There is nothing worse than to get your old window out and then a downpour starts. Start by removing the old window glass. This may be easy if you have newer style windows, or difficult if you have the older style with individual panes. In either case, the use of work gloves is a prudent precaution. In extreme cases, you may have to bust the glass out.

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Using an assortment of pry bars, hammers, and chisels, remove the window frame. Again, this can be relative easy or a royal pain. You won't be saving any parts, so go at it with hammer and chisel and pry bars until all you have all around the opening is what ever your home is made of.

Step 4: Test Place Your Window

This is where a helper is really handy. Lift your new glass block unit and place it in the opening. Ideally, you should have about an inch or so all around (two inches on top as your new window will be resting flush against the bottom while test fitting).

Important: Take note of which way the casement window open. Many older types open outward while most newer types open inward. Ensure that the latches or crank to open your new window are facing inside. Once your window is in, it's in.

Window with frame removed.

Window with frame removed.

Step 5: Mix Up the Mortar

Tip: DO NOT use quick setting mortar. You want as much time a possible to get this right, and quick setting mortar is unforgiving.

Mix the mortar according to the manufacturer's instructions. While a bucket will work, I find it easier to use something like a cat's (clean) litter box. After the mortar is the right consistency, lay a two-inch thick bead of mortar along the bottom. Put two shims on the bead and push them down into the mortar about an inch. Put the shims in so the thin end is just extended on the other side of the bead.

Step 6: Place the Window

Tip: Most basement windows are near level with the ground, but some have a bit of a drop. Use your helper or place something on the other side to ensure your window does not fall out when placing it.

Set the new window on top of the bead of mortar. Check for level and tap the shims with a hammer to level the window. Use your trowel or putty knife to trim away the excess mortar that has squished out at the bottom, leaving enough to make a nice foundation. Use your towel or putty knife to smooth the mortar at the bottom. Do this on both sides.

I built a simple brace to keep window in place while adding mortar.

I built a simple brace to keep window in place while adding mortar.

Step 7: Fill in the Gaps

Using the leftover mortar, and wearing nitrile gloves, use your putty knife and fingers to push in mortar on the sides and top. Don't forget to do this on both sides. It is better to have to mix two batches of mortar than to have what you mixed up begin to harden midway through the project.

Once the gaps are filled, smooth out the exposed mortar using the tool of your choice (most likely your fingers). Check for level once again and adjust if necessary by tapping on the shims. After about 45 minutes, you can snap off the exposed portion of the shims. Mix a very small batch of mortar and cover the shim area.

Use the rest to fill in any gaps you missed. Also check the slant of any exposed window sill on the outside. Ensure that it slants away from the house, if not, mix up some mortar and apply a thin coat and shape it so that it will slant towards the outside.

Level and set window.

Level and set window.

Step 8: Fill in the Voids

Depending on how your home is built, there may be some large voids around where you placed your window. Put on a new pair of nitrile gloves and use the expanding spray foam to fill them in. After the foam has dried, trim with a utility knife for a neat presentation.

Step 9: Seal the Seal

Give the mortar a week to completely cure. Then brush on a liquid sealant. Any type that is rated for cement will work. While not necessary, you can also put a bead of outdoor rated acrylic sealant where the glass meets the mortar.

Perfect? No. Functional and satisfying? Yes.

Perfect? No. Functional and satisfying? Yes.

Step 10: Enjoy the Light and Air

You new glass block windows will let in a ton of natural light, keep the basement warmer in winter, and help keep the stale are moving during warmer temperatures. And enjoy the extra security that the thick glass and solid mortar provides. Take pictures and send to your friends, proclaiming yourself to be king/queen of DIY.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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